1. Field of the Invention
The present invention relates generally to the area of fluid handling and solid/liquid separation and, more particularly, to a scroll and screen centrifuge having abrasion resistant scroll inserts and feed openings designed to reduce abrasion of the scroll.
2. Description of Related Art
It is well known to use a scroll and screen centrifuge to separate a crystalline, a granular or a fibrous product from a liquid in which the crystalline, granular or fibrous product is carried. Scroll and screen centrifuges are typically used with a continuous process in which a combined liquid and solid material is continuously fed into and continuously discharged from the centrifuge. The continuous process is distinguished from a batch process in which a batch of the liquid and solid material is fed into the centrifuge; and the liquid and solid materials are separated. The centrifuge is stopped, and the separated materials are removed before another batch is loaded into the centrifuge. Both the continuous and batch separation processes are also considered to dry the solid material. The separation process is used in a wide range of applications including the production of soap powders, coal, gypsum, plastic granules, potash, salt, etc.
In a typical scroll and screen centrifuge, a frustoconical scroll and cage assembly is mounted for rotation within a housing. A frustoconical scroll member having a tubular side wall with outwardly projecting helical screw flights is mounted for rotation within an independently rotating frustoconical screen cage, and a screen basket is mounted within the screen cage. The scroll member and cage member rotate at a slightly different angular velocities so that the helical screw flights on the scroll member pass close to and over an inner directed surface of the screen. The helical length of the screw flights varies in a range of from a fraction of a revolution to several revolutions around the scroll member. A liquid containing solids to be separated is fed into a closed rearward portion of the frustoconical scroll and cage assembly. The rotation of the screen cage produces a centrifugal force causing the liquid to pass through openings in the screen basket thereby separating the liquid from the solids. The rotational velocity difference between the scroll member and cage member causes the screw flights to push the solid material across the screen from the smaller, closed, rear end of the scroll and cage assembly to its larger, open, forward end from which the solid material is discharged. The processing time or retention time of the material in the centrifuge is controlled by the pitch of the helical screw flights on the scroll member and the difference in angular velocities between the scroll and cage members. Retention time is also influenced by the design of the feed openings in the scroll. Gravity pulls the solid particles through the plenum to another processing station, a conveyor or a collection unit.
The most vigorous interaction between the combined liquid and solid material occurs at the smaller, closed end of the scroll and cage assembly. At that point, the combined liquid and solid material passes from the interior of the scroll member through feed openings in the tubular side wall to a volume bounded by the end wall of the scroll member, the outer surface of the tubular side wall of the scroll member, the helical screw flights extending from the scroll member and the screen. As the combined liquid and solid material passes through the feed openings, it contacts the screw flights over their full width, which extends from the outer surface of the tubular side wall of the scroll member to the screen; and the combined liquid and solid material contact the screw flights over a significant portion of their length. In some applications, for example, the separation of coal from a coal and water slurry, the physical characteristics of the coal and slurry water, are such that vigorous contact of the coal slurry with the screw flights will result in the structural material of the screw flights being abraded away.
The rotation of the scroll and cage assembly is effective to quickly separate the liquid from the solid material after a translation of the combined liquid and solid material from the smaller, closed end of the scroll and cage assembly toward the larger, open discharge end. In a coal slurry, the greatest wear occurs at the feed area where the slurry impacts on the rotating scroll. In addition to the impact, the acceleration of the solids from a static state to a rotating state produces high abrasion. Motion of the solid material toward the discharge end is controlled by the difference in angular velocities between the scroll member and the screen cage. The abrading action is lower at the discharge end because the relative velocities of the rotating components and the coal is less vigorous.
The extent and severity of the abrading action will depend on the physical characteristics, that is, the abrasion characteristics of the combined liquid and solid material, the design of the scroll and cage assembly and the operating parameters of the centrifuge. To reduce the wear and tear by the solid material on the screw flights of the scroll member, abrasion resistant elements comprising abrasion resistant tiles are bonded on top of the forward directed surfaces of the screw flights proximate the rear end of the scroll member. However, the bonding agent in the joints between the tiles and in the joints between the underside of the tiles and the surfaces of the screw flights projects above the surface of the screw flight and is subject to the same abrading action by the solid material. Consequently, the bonding agent experiences the same wear and tear from the abrading action of the solid material and is abraded away. Therefore, scroll members with such abrasion resistant elements have the disadvantage of that after a short period of use, for example, several months, the combined liquid and solid material abrades away the bonding agent in the joints between and below the tiles; and the tiles become separated from the top surfaces of the screw flights. Therefore, the useful life of the scroll member is shortened, and the centrifuge must be rebuilt with a replacement scroll member at a substantial expense.
The designs of prior scroll members have feed openings with lips adjacent the end wall that trap solids inside the scroll member. The trapped solids move in circular paths inside the scroll member and undercut the lip and abrade the adjacent end wall. Therefore, such designs have the disadvantage of having unnecessary abrasion and wear and tear on the scroll and cage assembly, thereby reducing its useful life.